1. Field of the Invention
The present invention relates to metal-organic microporous coordination polymers and their use in chemical separation of gases.
2. Background Art
The development of materials with high adsorption of carbon dioxide at low relative pressure and ambient temperature has been a significant challenge to materials chemistry. Physical adsorption of carbon dioxide is an emergent technology. This system may replace the currently used processes, such as chemical adsorption by a bed of amine, which are costly to operate. Microporous coordination polymers (MCPs) offer advantages as high surface area materials yet the challenge that is addressed here is the development of an MCP with a strong affinity for CO2. Such a material provides high adsorption at low relative pressures and ambient temperature. Amines are presently materials of choice for sequestration of CO2. Flue gas, for example from coal combustion, is bubbled through a solution of amine that reacts with the CO2. The product of this reaction can then be pumped to another vessel and heated to release the CO2 and recycle the amine. The process however is inefficient and costly in terms of energy required to recycle the amine. Physisorption, as opposed to this chemisorption, is likely to be a more efficient and less expensive method of sequestration of CO2 if high affinity CO2 materials are developed. In general, physisorption provides ready release of the sorbed gas with moderate changes in pressure or temperature unlike chemisorption which generally requires more vigorous conditions.
In light of this, adsorbent materials or molecular sieves such as zeolites have been investigated extensively for CO2 uptake. Zeolite syntheses often require high temperature conditions for both syntheses and calcinations. In most cases, the nature of zeolites does not provide the opportunity for synthetic flexibility and/or ready functionalization. Yet to this point zeolites have been among the best materials for CO2 uptake. Zeolite 13X (UOP) has been the traditionally used sorbent for CO2 storage and has been reported to provide uptake of 4.7 mmol CO2/g sorbent (20.7 wt %) at 1 bar and 298 K.
A new class of materials known as MCPs offer many advantages to zeolites in the ease of synthesis, flexibility in functionalization and alteration, and characterization due to crystallinity. MCPs are composed of multifunctional organic linkers and metal-containing secondary building units (SBUs). Synthetic procedures are easily altered by modification of the linker such that functional groups can be added to the framework or the framework can be expanded. Substitution of the metal in the synthesis of MCPs has generally led to different SBUs which in turn leads to global changes to the resulting structure and porosity. Thus, careful examination of the metal effects alone has not been possible due to differences in structure of the resulting networks. There is a need to examine the effects of only the metal on the resulting properties of the MCP. Here isostructural MCPs have been synthesized and the effect on CO2 uptake is investigated leading to a very high affinity CO2 material.
Currently known MCPs have relatively unexceptional uptake of CO2 at low relative pressure. The best performance achieved previously by a material in this class is 21.4 wt % at 1.1 bar and 298 K by MOF-74. This material is difficult to synthesize and activate to high surface area. Also, the activation/evacuation conditions required to achieve porosity are relatively harsh (vacuum, 270° C., 16 hours). HKUST-1 [Cu3(BTC)2, BTC=benzene-1,3,5-tri-carboxylate or trimesylate] is another MCP with relatively strong affinity for CO2. The adsorption of CO2 on HKUST-1 was first reported in 2002 to achieve ˜4.2 mol CO2/kg MCP or ˜18 wt % at ˜1 bar and 295 K. This was substantiated in 2005 by a report of 17.9 wt % at 1 bar and 298 K. The most recent report of a relatively strong affinity CO2 material was in 2007 using Fe4O2(BTB)8/3, which displayed ˜95 mL CO2/g or ˜19 wt % at 273 K and ˜48 mL CO2/g or ˜9.4 wt % at 298 K. Thus, currently known MCPs are comparable to 13× zeolite in low pressure CO2 uptake at room temperature and there remains a great need for higher affinity CO2 uptake materials and especially those that function well below one bar.
The separations of close boiling mixtures of ethane/ethylene or propane/propylene are among the most energy-intensive separations carried out in the chemical and petrochemical industry. Because of the relatively close boiling points within the two pairs of compounds, cryogenic distillation at super pressures in trayed fractionators is still used to separate them on a large scale. Other approaches that are potentially less energy intensive involve an absorption/stripping method based on aqueous silver nitrate solutions and adsorption on a solid sorbent. Zeolites or molecular sieves and π-complexation sorbents are types of sorbents have been examined to accomplish these separations. Among the former materials that have been studied are activated carbon, 4A zeolite, 13× molecular sieve and the aluminophosphate AlPO4-14 molecular sieve. π-Complexation sorbents are normally based on silica gel and activated alumina that are impregnated with transition metal salts such as AgNO3 or CuCl. Other materials that have been examined for light olefin/paraffin separations are metal-containing facilitated transport membranes.